1. Field of the Invention
This invention relates to the field of reliability test systems, and more particularly to a test system that tests the reliability of optical devices such as diode lasers, and more specifically, tunable lasers.
2. Description of Related Art
Laser diodes are a substantial and fast growing constituent of optical communications networks. Such optical systems include, but are not limited to, telecommunication systems, cable television systems, and Local Area Networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, N.Y.) c. 1993 and Agrawal, G. Fiber-Optic Communications System, Wiley, 1997, the disclosures of which are incorporated herein by reference. Currently, the majority of optical systems are configured to carry an optical channel of a single wavelength over one or more optical wave-guides. To convey the information form plural sources, time division multiplexing is frequently employed (TDM). In time division multiplexing, a particular time slot is assigned to each information source, the complete signal being constructed from the signal collected from each time slot. While this is a useful technique for carrying plural information sources on a s single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication systems increases, the current capacity of existing wave-guiding media is limited. Although capacity may be expanded, e.g. by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of the existing optical wave-guides.
Wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) have been explored as approaches for increasing the capacity of the existing fiber optic networks. Such systems employ plural optical signal channels, each channel being assigned a particular channel wavelength. In a typical system, optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single wave-guide, and de-multiplexed such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM and DWDM approaches in long distance optical systems.
The desire to harvest the benefits offered by these new technologies requires the incorporation of rapidly evolving state-of-the art components for which there has been no prior use and about which little is known in detail. Substantial emphasis must then be placed upon strategies for the early detection of system-threatening premature failures that may occur. These strategies are based upon a priori considerations, rather than extensive manufacturing and field experience. There has to be as much focus upon the possible as upon the probable failure modes. Since WDM and DWDM are driving exponential increase in demand for the components, dramatic shift in manufacturing techniques to achieve higher yields and improve device reliability are required.
Discrete diode lasers for use in telecommunications equipment are typically coupled to fiber-optic amplifiers. The expected operating life of telecommunications equipment is over 20 years, necessitating implementation of a protocol assuring the reliability of diode lasers, subassemblies, and ultimately of the entire telecommunications systems. Typically, the laser diodes are intended for use in an environment whose temperature is as high as 85 Degrees Celsius. The performance of the diode lasers varies with temperature. Typically, peak wavelength, threshold current and operating current will shift by certain amount at elevated temperatures. The telecommunications systems are designed to accommodates certain amount of shift in laser parameters and still perform satisfactorily. Some of the devices, however, will continue to shift during the operation until they ultimately fail, and also cause the ultimate failure of the communications system. The system requirements, therefore, necessitate that diode lasers pass certain reliability criteria that will assure a minimal acceptable drift in their parameters and provide satisfactory performance over the long life of the communications system. The purpose of reliability testing, therefore, is to screen out and reject the devices that would not pass the required criteria. As an example of the approach employed, a typical diode laser power v. current behavior prior to and post burn in is shown in FIG. 1(a). The shift shown, if beyond the system specification, will cause the device to be rejected.
Traditionally, laser testing and burn-in were primarily accomplished by numerous manual steps which required significant operator to test equipment interactions. Most laboriously, the photodetector calibration, required to be done at start of each measurement, when done manually, is very time consuming and needs to be repeated with regularity. The data collection schemes involved either manual recording systems or stand alone non communicating individual test systems. The traceability of data had to be accomplished manually and remote accessibility of test data for future failure analysis were virtually non existent.
There is a need for a test system that eliminates virtually all the labor intensive steps, automatically performs laser to photodetector calibration and enables all system components to function together.